Experiments with Alternate Currents of High Potential and High Frequency (1892) — part 7 of 8

a high degree of incandescence; but as to the nature of the forces there would be considerable uncertainty, for it would be doubtful whether with such impulses the copper core would act as a static screen. Such paradoxes and apparent impossibilities we encounter at every step in this line of work, and therein lies, to a great extent, the cl aim of the study.

I have here a short and wide tube which is exhausted to a high . degree and covered with a substantial coating of bronze, the coating allowing barely the light to shine through. A metallic clasp, with a hook for suspending the tube, is fastened around the middle portion of the latter, the clasp being in contact with the bronze coating. I now want to light the gas inside by suspending the tube on a wire connected to the coil. Any one who would try the experiment for the first time, not having any previous ex¬ perience, would probably take care to be quite alone when ' making the trial, for fear that he might become the joke of his assistants. Still, the bulb lights in spite of the metal . coating, and the light can be distinctly perceived through the latter. A long tube covered with aluminium bronze lights when held in one hand — the other touching the terminal of the coil — quite powerfully. It might be objected that the coatings are not sufficiently conducting ; still, even if they were highly resistant, they ought to screen the gas. They ceitainly screen it perfectly in a condition of rest, but not by far perfectly when the charge is surging in the coating. But the loss of energy which occurs within the tube, notwithstanding the screen, is occasioned principally by the presence of the gas. Were

128

we to take a large hollow metallic sphere and fill it with a perfect incompressible fluid dielectric, there would be no loss inside of the sphere, and consequently the inside might be considered as perfectly screened, though the potential be very rapidly alternating. Even were the sphere filled with oil, the loss would be incomparably smaller than when the fluid is replaced by a gas, for in the latter case the force produces displacements; that means impact and collisions in the inside.

No matter what the pressure of the gas may be, it be¬ comes an important factor in the heating of a conductor when the electric density is great and the frequency very high. That in the heating of conductors by lightning dis¬ charges air is an element of great importance, is almost as certain as an experimental fact. I may illustrate the action of the air by the following experiment: I take a short tube which is exhausted to a moderate degree and has a platinum wire running through the middle from one end to the other. I pass a steady or low frequency current through the wire, and it is heated uniformly in all parts. The heating here is due to conduction, or frictional losses, and the gas around the wire has — as far as we can see — no function to perform. But now let me pass sudden dis¬ charges, or a high frequency current, through the wire. Again the wire is heated, this time principally on the ends and least in the middle portion; and if the frequency of the impulses, or the rate of change, is high enough, the wire might as well be cut in the middle as not, for practically all the heating is due to the rarefied gas. Here the gas might only act as a conductor of no impedance

129

diverting the current from the wire as the impedance of the latter is enormously increased, and merely heating the ends of the wire by reason of their resistance to the passage of the discharge. But it is not at all necessary that the gas in the tube should be conducting; it might be at an ex¬ tremely low pressure, still the ends cf the wire would be heated — as, however, is ascertained by experience — only the two ends would in such case not be electrically con¬ nected through the gaseous medium. Now what with these frequencies and potentials occurs in an exhausted tube occurs in the lightning discharges at ordinary pressure. We only need remember one of the facts arrived at in the course of these investigations, namely, that to impulses of very high frequency the gas at ordinary pressure behaves much in the same manner as though it were at moderately low pressure. I think that in lightning discharges fre¬ quently wires or conducting objects are volatilized merely because air is present, and that, were the conductor im¬ mersed in an insulating liquid, it would be safe, for then the energy would have to spend itself somewhere else. From the behavior of gases to sudden impulses of high po¬ tential I am led to conclude that there can be no surer way of diverting a lightning discharge than by affording it a passage through a volume of gas, if such a thing can be done in a practical manner.

There are two more features upon which I think it neces¬ sary to dwell in connection with these experiments— the “ radiant state ” and the “ non-striking vacuum.”

Any one who has studied Crookes’ work must have re¬ ceived the impression that the “ radiant state ” is a property

130

of the gas inseparably connected with an extremely high degree of exhaustion. But it should be remembered that the phenomena observed in an exhausted vessel are limited to the character and capacity of the apparatus which is made use of. I think that in a bulb a molecule, or atom,

Fig. 31. -Bulb Showing Radiant Lime Stream at Low

Exhaustion.

does not precisely move in a straight line because it meets no obstacle, but because the velocity imparted to it is suffi¬ cient to propel it in a sensibly straight line. The mean free path is one thing, but the velocity — the energy associated

i3i

with the moving body — is another, and under ordinary cir¬ cumstances I believe that it is a mere question of potential or speed. A disruptive discharge coil, when the potential is pushed very far, excites phosphorescence and projects shadows, at comparatively low degrees of exhaustion. In a lightning discharge, matter moves in straight lines at ordinary pressure when the mean free path is" exceedingly small, and frequently images of wires or other metallic objects have been produced by the particles thrown off in straight lines.

I have prepared a bulb to illustrate by an experiment the correctness of these assertions. In a globe L (Fig. 31, I have mounted upon a lamp filament / a piece of lime l. The lamp filament is connected with a wire which leads into the bulb, and the general construction of the latter is as indicated in Fig. 19, befcie described. The bulb being suspended from a wire connected to the terminal of the coil, and the latter being set to work, the lime piece Z and the projecting parts of the filament / are bombarded. The degree of exhaustion is just such that with the potential the coil is capable of giving phosphorescence of the glass is produced, but disappears as soon as the vacuum is impaired. The lime containing moisture, and moisture being given off as soon as heating occurs, the phospho¬ rescence lasts only for a few moments. When the lime has been sufficiently heated, enough moisture has been given off to impair materially the vacuum of the bulb. As the bombardment goes on, one point of the lime piece is more heated than c,ther points, and the result is that finally practically all the discharge passes through that

182

point which is intensely heated, and a white stream of lime particles (Fig. 31) then breaks forth from that point. This stream is composed of “ radiant” matter, yet the degree of exhaustion is low. But the particles move in straight lines because the velocity imparted to them is great, and this is due to three causes — to the great electric density, the high temperature of the small point, and the fact that the par¬ ticles of the lime are easily torn and thrown off — far more easily than those of carbon. With frequencies such as we are able to obtain, the particles are bodily thrown off and projected to a considerable distance; but with sufficiently high frequencies no such thing would occur: in such case only a stress would spread or a vibration would be propa¬ gated through the bulb. It would be out of the question to reach any such frequency on the assumption that the atoms move with the speed of light; but I believe that such a thing is impossible; for this an enormous potential would be required. With potentials which we are able to obtain, even with a disruptive discharge coil, the speed must be quite insignificant.

As to the “ non-striking vacuum,” the point to be noted is that it can occur only with low frequency impulses, and it is necessitated by the impossibility of carrying off enough energy with such impulses in high vacuum since the few atoms which are around the terminal upon coming in con¬ tact with the same are repelled and kept at a distance for a comparatively long period of time, and not enough work can be performed to render the effect perceptible to the eye. If the difference of potential between the terminals is raised, the dielectric breaks down. But with very high

133

frequency impulses there is no necessity for such breaking down, since any amount of work can be performed by con¬ tinually agitating the atoms in the exhausted vessel, provided the frequency is high enough. It is easy to reach — even with frequencies obtained from an alternator as here used — a stage at which the discharge does not pass between two electrodes in a narrow tube, each of these being connected to one of the terminals of the coil, but it is difficult to reach a point at which a luminous discharge would not occur around each electrode.

A thought which naturally presents itself in connection with high frequency currents, is to make use of their pow¬ erful electro-dynamic inductive action to produce light effects in a sealed glass globe. The leading-in wire is one of the defects of the present incandescent lamp, and if no other improvement were made, that imperfection at least should be done away with. Following this thought, I have carried on experiments in various directions, of which some were indicated in my former paper. I may here mention one or two more lines of experiment which liave been followed up.

Many bulbs were constructed as shown in Fig. 32 and Fig. 83.

In Fig. 32 a wide tube T was sealed to a smaller W- shaped tube U, of phosphorescent glass. In the tube T was placed a coil C of aluminium wire, the ends of which were provided with small spheres t and tx of aluminium, and reached into the U tube. The tube T was slipped into a socket containing a primary coil through which usually the discharges of Ley den jars were directed, and

134

the rarefied gas in the small V tube was excited to strong luminosity by the high-tension currents induced in the coil C. When Leyden jar discharges were used to induce cur-

Fig. 32.— Electro-Dynamic Fig. 33. — Electro-Dynamic Induction Tube. Induction Lamp.

rents in the coil O, it was found necessary to pack the tube T tightly with insulating powder, as a discharge would occur frequently between the turns of the coil, especially

185

when the primary was thick and the air gap, through which the jais discharged, large, and no little trouble was experienced in this way.

In Fig. 83 is illustrated another form of the bulb con¬ structed. In this case a tube T is sealed to a globe L. The tube contains a coil C, the ends of which pass through two small glass tubes t and tx, which are sealed to the tube T. Two refractory buttons m and m± are mounted on lamp filaments which are fastened to the ends of the wires passing through the glass tubes t and tx. Generally in bulbs made on this plan the globe L communicated with the tube T. For this purpose the ends of the small tubes t and tx were just a trifle heated in the burner, merely to hold the wires, but not to interfere with the communica¬ tion. The tube T, with the small tubes, wires through the same, and the refractory buttons m and ml, was first pre¬ pared, and then sealed to globe L, whereupon the coil C was slipped in and the connections made to its ends. The tube was then packed with insulating powder, jamming the latter as tight as possible up to very nearly the end, then it was closed and only a small hole left through which the remainder of the powder was introduced, and finally the end of the tube was closed. Usually in bulbs constructed as shown in Fig. 33 an aluminium tube a was fastened to the upper end s of each of the tubes t and tlt in order to protect that end against the heat. The buttons m and m1 could be brought to any degree of incandescence by passing the dis¬ charges of Leyden jars around the coil C. In such bulbs with two buttons a very curious effect is produced by the formation of the shadows of each of the two buttons.

136

Another line of experiment, which has been assiduously followed, was to induce by electro- dynamic induction a current or luminous discharge in an exhausted tube or bulb. This matter has received such able treatment at the hands of Prof. J. J. Thomson that I could add but little to what he has made known, even had I made it the special subject of this lecture. Still, since experiences in this line have gradually led me to the present views and results, a few words must be devoted here to this subject.

It has occurred, no doubt, to many that as a vacuum tube is made longer the electromotive force per unit length of the tube, necessary to pass a luminous discharge through the latter, gets continually smaller; therefore, if the ex¬ hausted tube be made long enough, even with low fre¬ quencies a luminous discharge could be induced in such a tube closed upon itself. Such a tube might be placed around a hall or on a ceiling, and at once a simple ap¬ pliance capable of giving considerable light would be obtained. But this would be an appliance hard to manu¬ facture and extremely unmanageable. It would not do to make the tube up of small lengths, because there would be with ordinary frequencies considerable loss in the coatings, and besides, if coatings wrere used, it would be better to supply the current directly to the tube by con¬ necting the coatings to a transformer. But even if all objections of such nature were reimned, still, with low fre¬ quencies the light conversion itself would be inefficient, as I have before stated. In using extremely high frequencies the length of the secondary — in other words, the size of the vessel— can be reduced as far as desired, and the effi-

137

ciency of the light conversion is increased, provided that means are invented for efficiently obtaining such high fre¬ quencies. Thus one is led, from theoretical and practical considerations, to the use of high frequencies, and this means high electromotive forces and small currents in the primary. When he works with condenser charges — and they are the only means up to the present known for reaching these extreme frequencies — he gets to electro¬ motive forces of several thousands of volts per turn of the primary. He cannot multiply the electro-dynamic induct¬ ive effect by taking more turns in the primary, for he ar¬ rives at the conclusion that the best way is to work with one single turn — though he must sometimes depart from this rule — and he must get along with whatever inductive effect he can obtain with one turn. But before he has long experimented with the extreme frequencies required to set up in a small bulb an electromotive force of several thousands of volts he realizes the great importance of elec¬ trostatic effects, and these effects grow relatively to the electro-dynamic in significance as the frequency is in¬ creased.

Now, if anything is desirable in this case, it is to increase the frequency, and this would make it still worse for the electro-dynamic effects. On the other hand, it is easy to exalt the electrostatic action as far as one likes by taking more turns on the secondary, or combining self-induction and capacity to raise the potential. It should also be remem¬ bered that, in reducing the current to the smallest value and increasing the potential, the electric impulses of high fre¬ quency can be more easily transmitted through a conductor.

138

These and similar thoughts determined me to devote more attention to the electrostatic phenomena, and to en¬ deavor to produce potentials as high as possible, and alter¬ nating as fast as they could be made to alternate. I then found that I could excite vacuum tubes at considerable distance from a conductor connected to a properly con¬ structed coil, and that I could, by converting the oscilla¬ tory current of a condenser to a higher potential, establish electrostatic alternating fields which acted through the whole extent of a room, lighting up a tube no matter where it was held in space. I thought I recognized that I had made a step in advance, and I have persevered in this line; but I wish to say that I share with all lovers cf science and progress the one and only desire — to reach a result of utility to men in any direction to which thought or experi¬ ment may lead me. I think that this departure is the right one, for I cannot see, from the observation of the phenom¬ ena which manifest themselves as the frequency is in¬ creased, what there would remain to act between two circuits conveying, for instance, impulses of several hundred millions per second, except electrostatic forces. Even with such trifling frequencies the energy would be practically all potential, and my conviction has grown strong that, to whatever kind of motion light may be due, it is produced by tremendous electrostatic s resses vibrating with extreme rapidity.

Of all these phenomena observed with currents, or electric impulses, of high frequency, the most fascinating for an audience are certainly thos$ which are noted in an electro¬ static field acting through considerable distance, and the

139

best an unskilled lecturer can do is to begin and finish with the exhibition of these singular effects. I take a tube in the hand and move it about, and it is lighted wherever I may hold it ; throughout space the invisible forces act. But I may take another tube and it might not light, the vacuum being very high. I excite it by means of a dis¬ ruptive discharge coil, and now it will light in the electro¬ static field. I may put it away for a few weeks or months, still it retains the faculty of being excited. What change have I produced in the tube in the act of exciting it? If a motion imparted to the atoms, it is difficult to perceive how it can persist so long without being arrested by frictional losses ; and if a strain exerted in the dielectric, such as a simple electrification would produce, it is easy to see how it may persist indefinitely, but very difficult to understand why such a condition should aid the excitation when we have to deal with potentials which are rapidly alternating.

Since I have exhibited these phenomena for the first time, I have obtained some other interesting effects. For in¬ stance, I have produced the incandescence of a button, filament, or wire enclosed in a tube. To get to this result it was necessary to economize the energy which is obtained from the field and direct most of it on the small body to be rendered incandescent. At the beginning the task appeared difficult, but the experiences gathered permitted me to reach the result easily. In Fig. 34 and Fig. 35 two such tubes are illustrated which are prepared for the occasion. In Fig. 34 a short tube Tx , sealed to another long tube T, is pro¬ vided with a stem s, with a platinum wire sealed in the latter. A very thin lamp filament Z is fastened to this

140

wire, and connection to the outside is made through a thin copper wire w. The tube is provided with outside and inside coatings, C and G\ respectively, and is filled as far as

Fig. 34.— Tube with Fila- Fig. 35. — Crookes’ Experi¬ ment Rendered Incan- ment in Electrostatic

DESCENT IN AN ELECTRO FIELD.

static Field.

the coatings reach with conducting, and the space above with insulating powder. These coatings are merely used to enable me to perform two experiments with the tube —

141

namely, to produce the effect desired either by direct con¬ nection of the body of the experimenter or of another body to the wire w, or by acting inductively through the glass. The stem sis provided with an aluminium tube a, for pur¬ poses before explained, and only a small part of the fila¬ ment reaches out of this tube. By holding the tube T ^ any¬ where in the electrostatic field the filament is rendered in¬ candescent.

A more interesting piece of apparatus is illustrated in Fig. 35. The construction is the same as before, only in¬ stead of the lamp filament a small platinum wire p, sealed in a stem s, and bent above it in a circle, is connected to the copper wfire wThich is joined to an inside coating C. A small stem is provided with a needle, on the point of which is arranged to rotate very freely a very light fan of mica v. To prevent the fan from falling out, a thin stem of glass g is bent properly and fastened to the alu¬ minium tube. When the glass tube is held anywhere in the electrostatic field the platinum wire becomes incandescent, and the mica vanes are rotated very fast.

Intense phosphorescence may be excited in a bulb by merely connecting it to a plate within the field, and the plate need not be any larger than an ordinary lamp shade. The phosphorescence excited with these currents is incom¬ parably more powerful than with ordinary apparatus. A small phosphorescent bulb, when attached to a wire con¬ nected to a coil, emits sufficient light to allow reading or¬ dinary print at a distance of five to six paces. It was of interest to see how some of the phosphorescent bulbs of Professor Crookes would behave with these currents, and

142

he has had the kindness to lend me a few for the occasion. The effects produced are magnificent, especially by the sul¬ phide of calcium and sulphide of zinc. From the disrup¬ tive discharge coil they glow intensely merely by holding them in the hand and connecting the body to the terminal of tliec ail.

To whatever results investigations of this kind may lead, their chief interest lies for the present in the possibilities they offer for the production of an efficient illuminating device. In no branch of electric industry is an advance more desired than in the manufacture of light. Every thinker, when considering the barbarous methods em¬ ployed, the deplorable losses incurred in our best systems of light production, must have asked himself, What is likely to be the light of the future? Is it to be an incandescent solid, as in the present lamp, or an incandescent gas, or a phosphorescent body, or something like a burner, but in¬ comparably more efficient ?

There is little chance to perfect a gas burner ; not, per¬ haps, because human ingenuity has been bent upon that problem for centuries without a radical departure having been made — though this argument is not devoid of force— but because in a burner the higher vibrations can never be reached except by passing through all the low ones. For how is a flame produced unless by a fall of lifted weights? Such process cannot be maintained without renewal, and renewal is repeated passing from low to high vibrations. One way only seems to be open to improve a burner, and that is by trying to reach higher degrees of incandescence. Higher incandescence is equivalent to a quicker vibration;

143

that means more light from the same material, and that, again, means more economy. In this direction some im¬ provements have been made, but the progress is hampered by many limitations. Discarding, then, the burner, there remain the three ways first mentioned, which are essen¬ tially electrical.

Suppose the light of the immediate future to be a solid rendered incandescent by electricity. Would it not seem that it is better to employ a small button than a frail fila¬ ment ? From many considerations it certainly must be concluded that a button is capable of a higher economy, assuming, of course, the difficulties connected with the operation of such a lamp to be effectively overcome. But to light such a lamp we require a high potential ; and to get this economically we must use high frequencies.

Such considerations apply even more to the production of light by the incandescence of a gas, or by phosphores¬ cence. In all cases we require high frequencies and high potentials. These thoughts occurred to me a long time ago.

Incidentally we gain, by the use of very high frequen¬ cies, many advantages, such as a higher economy in the light production, the possibility of working with one lead, the possibility of doing away with the leading-in wire, etc.

The question is, how far can we go with frequencies ? Ordinary conductors rapidly lose the facility of transmit¬ ting electric impulses when the frequency is greatly in¬ creased. Assume the means for the production of impulses of very great frequency brought to the utmost perfection, every one will naturally ask how to transmit them when the necessity arises. In transmitting such impulses through

144

conductors we must remember that we have to deal with pressure and flow, in the ordinary interpretation of these terms. Let the pressure increase to an enormous value, and let the flow correspondingly diminish, then such im¬ pulses — variations merely of pressure, as it were — can no doubt be transmitted through a wire even if their frequency be many hundreds of millions per second. It would, of course, be out of question to transmit such impulses through a wire immersed in a gaseous medium, even if the wire were provided with a thick and excellent insulation for most of the energy would be lost in molecular bom¬ bardment and consequent heating. The end of the wire connected to the source would be heated, and the remote end would receive but a trifling part of the energy sup¬ plied. The prime necessity, then, if such electric impulses are to be used, is to find means to reduce as much as pos¬ sible the dissipation.

The first thought is, employ the thinnest possible wire surrounded by the thickest practicable insulation. The next thought is to employ electrostatic screens. The insu¬ lation of the wire may be covered with a thin conducting coating and the latter connected to the ground. But this would not do, as then all the energy would pass through the conducting coating to the ground and nothing would get to the end of the wire. If a ground connection is made it can only be made through a conductor offering an enor¬ mous impedance, or though a condenser of extremely small capacity. This, however, does not do away with other difficulties.

If the wave length of the impulses is much smaller than

145

the length of the wire, then corresponding short waves wTill be sent up in the conducting coating, and it will be more or less the same as though the coating were directly connected to earth. It is therefore necessary to cut up the coating in sections much shorter than the wavelength. Such an arrangement does not still afford a perfect screen, but it is ten thousand times better than none. I think it preferable to cut up the conducting coating in small sec¬ tions, even if the current waves be much longer than the coating. •

If a wire were provided with a perfect electrostatic screen, it would be the same as though all objects were removed from it at infinite distance. The capacity would then be reduced to the capacity of the wire itself, which would be very small. It would then be possible to send over the wire current vibrations of very high frequencies at enormous distance without affecting greatly the char¬ acter of the vibrations. A perfect screen is of course out of the question, but I believe that with a screen such as I have just described telephony could be rendered practicable across the Atlantic. According to my ideas, the gutta¬ percha covered wire should be provided with a third con¬ ducting coating subdivided in sections. On the top of this should be again placed a layer of gutta-percha and other insulation, and on the top of the whole the armor. But such cables will not be constructed, for ere long in¬ telligence-transmitted without wires— will throb through the earth like a pulse through a living organism. The wonder is that, with the present state of knowledge and the experiences gained, no attempt is being made to dis-

146

turb the electrostatic or magnetic condition of the earth, and transmit, if nothing else, intelligence.

It has been my chief aim in presenting these results to point out phenomena or features of novelty, and to advance ideas which I am hopeful will serve as starting points of new departures. It has been my chief desire this evening to entertain you with some novel experiments. Your ap¬ plause, so frequently and generously accorded, has told me that I have succeeded.

In conclusion, let me thank you most heartily for your kindness and attention, and assure you that the honor I have had in addressing such a distinguished audience, the pleasure I have had in presenting these results to a gather¬ ing of so many able men — and among them also some of those in whose w'ork for many years past I have found en¬ lightenment and constant pleasure— I shall never forget.

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